1. Field of the Invention
The present invention relates to the fabrication of ultrathin multilayered films on suitable surfaces by electrostatic layer-by-layer self assembly (“ELBL”). More specifically, the present invention relates to a method for designing polypeptides for the nanofabrication of thin films, coatings, and microcapsules by ELBL for applications in biomedicine and other fields.
2. Description of Related Art
ELBL is an established technique in which ultrathin films are assembled by alternating the adsorption of oppositely-charged polyelectrolytes. The process is based on the reversal of the surface charge of the film after the deposition of each layer. FIG. 1 shows a schematic diagram of the general ELBL process: films of oppositely charged polyions (cationic polyions 10 and anionic polyions 11) are assembled in successive layers on a negatively-charged planar surface 12; the surface charge is reversed after the deposition of each layer. This process is repeated until a film of desired thickness is formed. The physical basis of association is electrostatics—gravitation and nuclear forces play effectively no role—and the increase in entropy on release of counterions into solution. Because of the generality and relative simplicity of the process, ELBL allows for the deposition of many different types of materials onto many different types of surface. There are, therefore, a vast number of possible useful combinations of materials and surfaces. For a general discussion of ELBL, including its history, see Yuri Lvov, “Electrostatic Layer-by-Layer Assembly of Proteins and Polyions” in Protein Architecture: Interfacial Molecular Assembly and Immobilization Biotechnology, Y. Lvov & H. Möhwald eds. (New York: Marcel Dekker, 1999), pp. 125-167, which is incorporated herein by reference in its entirety.
ELBL has recently become a focus area in the field of nanotechnology because it can be used to fabricate films substantially less than 1 micron in thickness. Moreover, ELBL permits exceptional control over the film fabrication process, enabling the use of nanoscale materials and permitting nanoscale structural modifications. Because each layer has a thickness on the order of a few nanometers or less, depending on the type of material used and the specific adsorption process, multilayer assemblies of precisely repeatable thickness can be formed.
A number of synthetic polyelectrolytes have been employed in ELBL applications, including sodium poly(styrene sulfonate) (“PSS”), poly(allylamine hydrochloride) (“PAH”), poly(diallyldimethylammonium chloride) (“PDDA”), poly(acrylamide-co-diallyldimethylammonium chloride), poly(ethyleneimine) (“PEI”), poly(acrylic acid) (“PAA”), poly(anetholesulfonic acid), poly(vinyl sulfate) (“PVS”), and poly(vinylsulfonic acid). Such materials, however, are not generally useful for biomedical applications because they are antigenic or toxic.
Proteins, being polymers with side chains having ionizable groups, can be used in ELBL for various applications, including biomedical ones. Examples of proteins that have been used in ELBL include cytochrome c, hen egg white lysozyme, immunoglobulin G, myoglobin, hemoglobin, and serum albumin (ibid.). There are, however, difficulties with using proteins for this purpose. These include limited control over multilayer structure (because the surface of the protein is highly irregular and proteins will not ordinarily adsorb on a surface in a regular pattern), restrictions on pH due to the pH-dependence of protein solubility and structural stability, lack of biocompatibility when using exogenous proteins, and the cost of scaling up production if the gene has not been cloned; unless the protein were identical in a readily available source, e.g. a cow, the protein would have to be obtained from the organism in which it was intended for use, making the cost of large-scale production of the protein prohibitive.
By contrast polypeptides, which are generally smaller and less complex than proteins, constitute an excellent class of material for ELBL assembly, and polypeptide film structures formed by ELBL will be useful in a broad range of applications. The present invention provides a method for designing polypeptides for the nanofabrication of thin films, coatings, and microcapsules by ELBL. Polypeptides designed using the method of the present invention should exhibit several useful properties, including, without limitation, completely determined primary structure, minimal secondary structure in aqueous solution, monodispersity, completely controlled net charge per unit length, ability to form cross-links on demand, ability to reverse cross-link formation, ability to form more organized thin films than is possible with proteins, and relatively inexpensive large-scale production cost (assuming gene design, synthesis, cloning, and host expression in E. coli or yeast, or peptide synthesis).
Polypeptides designed using the method of the present invention have been shown useful for ELBL of thin film structures with targeted or possible applications in biomedical technology, food technology, and environmental technology. Such polypeptides could be used, for example, to fabricate artificial red blood cells, drug delivery devices, and antimicrobial films.